Nanotechnology Spotlight

Nanocrystallizing implant surfaces to reduce biofilm infections

(Nanowerk Spotlight) Modern medicine would be unthinkable without biomedical implants. The market for medical implant devices in the U.S. alone is estimated to be $23 billion per year and it is expected to grow by about 10% annually for the next few years. Implantable cardioverter defibrillators, cardiac resynchronization therapy devices, pacemakers, tissue and spinal orthopedic implants, hip replacements, phakic intraocular lenses and cosmetic implants will be among the top sellers. Just consider the need for bone and dental implants: Each year, almost 500,000 patients receive hip implants worldwide, about the same number need bone reconstruction due to injuries or congenital defects and 16 million Americans loose teeth and may require dental implants. Current medical implants, such as orthopedic implants and heart valves, are made of titanium and stainless steel alloys, primarily because they are biocompatible.

Unfortunately, a common complication associated with medical implants results from infectious biofilms – multilayered colonies of bacteria that often form on implants – which may cause chronic infection that is difficult to control. For instance, biofilms are present on the teeth as dental plaque, where they may become responsible for tooth decay and gum disease. Because teeth are easily accessible, removing plaque is not a problem. If biofilms develop on medical implants deep inside the body, though, they can become a serious, sometimes life-threatening problem.

Preventing or limiting the formation of bacterial biofilm on the surface of implanted medical devices is an important approach to control bacterial biofilm-related infections. A new study demonstrates the effectiveness of a process combining surface nanocrystallization and thermal oxidation for reducing the biofilm’s adherence to stainless steel.

"In our recent study we demonstrate a surface treatment for stainless steel, one of the most common metallic biomedical materials, where we sandblast the target area and then turn the resultant dislocation cells in the surface layer into nanosized grains by a subsequent recovery treatment in air," Dr. Dongyang Li tells Nanowerk. "This process generated a more protective oxide film that blocked the electron exchange or reduced the surface activity more effectively. As a result, the biofilm's adherence to the treated surface was markedly minimized."

(A) A nanocrystallized surface annealed at 300 °C in air with its average grain size equal to 108 nm. (B) A nanocrystallized surface with its average grain size equal to 96 nm which was annealed at 250 °C. (C) A nanocrystallized surface with its average grain size equal to 32 nm when the annealed temperature is 200 °C. (D) A microcrystalline surface with an average grain size of around 40 µm. (Reprinted with permission from IOP Publishing)

Li, a professor in the Department of Chemical and Materials Engineering at the University of Alberta in Canada, explains that the passive film or a thermally formed oxide film on a material can reduce its reactivity.

"Such a function could be significantly enhanced when the substrate is nanocrystalline, since the associated high-density grain boundaries can considerably promote element diffusion (e.g., chromium diffusion along grain boundary), increase the oxide nucleation rate, and benefit the oxide adherence to the substrate" says Li. "All these could generate a stronger, more protective and stable oxide film."

Li explains that surface nanocrystallization of a metallic material has been demonstrated to be an effective approach for modifying the surface energy of metals. "The surface energy is directly related to surface activity, which can be characterized by the electron work function (EWF) that is the minimal energy required to move electrons from inside a metal to its surface," he says. "EWF is thus a characteristic parameter that reflects interactions between metals and foreign substances, including organisms. If a surface is active, the interaction between the material and the surrounding medium would be strong, resulting in a large adhesive force."

It was demonstrated already several years ago ("Surface nanocrystallization of 316L stainless steel induced by ultrasonic shot peening") that surface nanocrystallization could be achieved by ultrasonic shot peening treatment. For their study, Li and his team fabricated nanostructured stainless steel surfaces via a sandblasting and recovery process. They then measured the adhesive force between a biofilm and their stainless steel surface using an atomic force microscope (AFM). The AFM tip was coated with a synthetic peptide, a substitute of Pseudomonas aeruginosa biofilm, so that the interaction between the peptide and the steel surface could be evaluated.

For their study, Li and his team fabricated nanostructured stainless steel surfaces via a sandblasting and annealing process. They then measured the adhesive force between a biofilm and their stainless steel surface using an atomic force microscope (AFM). The AFM tip was coated with a synthetic peptide, a substitute of Pseudomonas aeruginosa biofilm, so that the interaction between the peptide and the steel surface could be evaluated.

"The AFM was used in the contact mode to determine the adhesive force between the AFM tip and a target surface" explains Li. "When the AFM tip is pulled away from the surface, the deflection of the cantilever reflects the adhesive force. The deflection of the cantilever is detected by a laser beam, from which the related force can be quantitatively determined if the spring constant of the cantilever is known."

The researchers found that the combined surface nanocrystallization and thermal oxidation provide an effective way to generate a more protective oxide film on stainless steel surfaces, which has a greater bond to the substrate, higher hardness, and a higher degree of inertness so as to prevent electrons reacting
with the surrounding medium. As a result of this surface treatment, the interaction between the peptide derived AFM tip and the stainless steel surface decreases.

"Our results demonstrate that the combination of surface nanocrystallization and thermal oxidation treatment is a promising approach to suppress the formation of infectious biofilms on metallic materials, thus providing a surface technique to minimize bacterial biofilms on implant surfaces for improved orthodontic and orthopedic applications" says Li.

This surface modification is promising not only for suppressing bacterial biofilms on medical implant materials but also has potential in treating materials for food processing and storage as well as for bio-corrosion control. Li points out that it is also possible to add additional elements into the nanocrystalline surface layer to further improve surfaces with anti-bacteria capability.